Flat display device is widely used due to thin body, power saving, no radiation and many other advantages. The existing flat display device includes mainly a liquid crystal display (LCD) and an organic light emitting display (OLED).
LCD display device is widely used in various consumer electronics such as mobile phones, televisions, personal digital assistants, digital cameras, notebook computers, and desktop computers for their high image quality, power saving, thin body and wide application advantages, thus becoming the mainstream in the display device.
OLED display device is an active light-emitting display with advantages of self-luminous, high contrast, wide viewing angle up to 170°, fast response, high luminous efficiency, low operating voltage (3V to 0V), and ultrathin thickness (less than 2 mm). Therefore, it has more excellent color display quality, wider viewing range and greater design flexibility.
Thin film transistor (TFT) is an important part of flat display device and can be formed on glass substrate or plastic substrate, it is commonly used as switching device and driving device in, for example, LCD display device and OLED display device.
Metal oxide semiconductor TFT is popular in current technology, metal oxide semiconductor due to its high electron mobility (oxide semiconductor mobility>10 cm2Ns, a-Si mobility of only 0.5˜0.8 cm2Ns), simple manufacturing process and high process compatibility with the a-Si TFT compared with low temperature polysilicon (LTPS), can therefore be applied to LCD display device and OLED display device. It is suitable for small to large size displays, has a good prospect of application development, and serves a popular research in the current industry.
Since the metal oxide semiconductor is relatively sensitive to light, the threshold voltage of the metal oxide semiconductor TFT is obviously negatively shifted after light irradiation. An improved method in the prior art is to disposed a metal light-shielding layer below an active layer of the metal oxide semiconductor to eliminate the TFT negative voltage drift caused by light. However, the metal light-shielding layer may cause some negative effects such as a floating gate effect. The floating gate effect means that the metal light-shielding layer disposed corresponding to the bottom of the active layer, therefore, is equivalent to a bottom gate. Although the metal light-shielding layer is not electrically connected to other charged layers in the TFT structure, it is easily affected by the voltage on these charged layers. Therefore, the metal light-shielding layer carries various voltages. Since the metal light-shielding layer has a fluctuating voltage, the threshold voltage of the TFT will constantly change during operation, resulting in unstable operation of the TFT.